MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020

Compounds and Stoichiometry

Chapter Profile


The content in this chapter should be relevant to about 8% of all questions about general chemistry on the MCAT.

This chapter covers material from the following AAMC content categories:

4C: Electrochemistry and electrical circuits and their elements

4E: Atoms, nuclear decay, electronic structure, and atomic chemical behavior

5A: Unique nature of water and its solutions


Oh—what is that smell? It smells like rancid almonds. Then you notice a few green bugs whose backs give the impression of a shield. Stink bugs! A stink bug “stinks” because it produces a highly concentrated solution of volatile compounds that we perceive as malodorous, noxious, and irritating. Interestingly enough, the primary compounds in the stink bug’s stink bomb are hydrogen cyanide—a highly toxic compound that inhibits cytochrome c oxidase, thereby blocking aerobic respiration—and benzaldehyde. Like many other aromatic compounds, benzaldehyde vaporizes at room temperature and reaches the olfactory system as gas particles. Benzaldehyde is also the key ingredient in artificial almond extract. At low concentrations, it produces a pleasant aroma of toasted almonds. However, at high concentrations, its odor is that of rotten almonds, and it is a noxious irritant to the skin, eyes, and respiratory tract.

Benzaldehyde is a compound composed of seven carbon atoms, six hydrogen atoms, and one oxygen atom. One mole of benzaldehyde has a mass of approximately 106 grams. It can react with other atoms or compounds to form new compounds—pure substances composed of two or more elements in a fixed proportion. Compounds can be broken down by chemical means to produce their constituent elements or other compounds. They are characterized by describing their physical and chemical properties.

This chapter focuses on compounds and their reactions. It reviews the various ways in which compounds are represented, using empirical and molecular formulas and percent composition. There is a brief overview of the major classes of chemical reactions, which we will examine more closely in subsequent chapters, and finally, there is a recap of the steps involved in balancing chemical equations with a particular focus on identifying limiting reagents and calculating reaction yields.